This invention is a guidebar for chainsaws of a kind comprising an elongated guidebar body and releasably secured to this a nose part. The guidebar body being provided with a groove running along its edge to receive and guide a saw chain. The nose part being secured to the guidebar body by riveting and having side plates supporting a nose sprocket. The side plates are shaped so that location of the nose part and the guidebar body are relative to each other, during and after riveting to provide an uninterrupted support for the saw chain.
In the front end of a guide bar where the saw chain travels around the nose part, it is common that one attempts to minimize friction and wear either by coating the guidebar with a material with high wear resistance, or by letting the chain be supported by a smooth or toothed nose sprocket. The toothed sprocket is preferred because one can be sure that it rotates even if its bearing runs stiffer than normal due to cold or fouling.
The nose part, the nose sprocket and its bearing are more exposed to damage than other parts of the guidebar, and there are many designs of guidebars, where the nose part including the nose sprocket and its bearing is made as a separate unit, secured to the remaining bar body with rivets. Examples of this are known from U.S. Pat. No. 596,802, No. 2,888,964 and No. 3,762,047. The rivets can easily be removed for change of nose part. Nose parts without sprockets can also be made as replaceable units as in Swedish Patent 201.979.
In all designs with a replaceable nose part, it is desirable with consideration of vibrations, wear and safety, that the saw chain is continuously guided by the guidebar without deviations from the plane of the guidebar, and that it is supported without interruptions, at least not along those parts of the guidebar where the saw chain can be engaged in cutting. Thus it is common that at the joint between the nose part and the guidebar body the contours of the plates involved are shaped to include some locating surfaces on tongues or recesses in the main direction of the guidebar. This minimizes the rise of dislocation during riveting, but this requires either very high precision for the lateral dimensions of the tongues, or a smoothing operation such as filing or grinding of the guidebar edge after riveting of the joint. Such designs for replaceable nose parts are known from U.S. Pat. Nos. 2,693,206, 2,888,964, 3,762,047, Swedish Patents 201.979, 433.468.
A different way to shape the locating surfaces is to let them form a large angle with the longitudinal axis of the guidebar, 45 degrees or more, as is shown in U.S. Pat. Nos. 3,955,279, and 4,489,493. This has the advantage of being rather insensitive to deviations in the shape of the contour, allowing the contour to be blanked with no finishing operation, or even that the plates of the nose part and the bar body are originally made as one integral piece, separated by blanking of the joint contour without loss of material, although both parts become slightly broadened by the slanting direction of the blanked surface. However, the larger angle has the disadvantage of imprecise locating at right angles to the guidebar axis, and the risk of sideways dislocation during riveting is great, especially by replacement, when the rivet holes might have become expanded. Because of this guidebars of these types must often be made with chamfered edges near the joint, which implies a length where the chain is inadequately supported.
A modification of the designs shown in U.S. Pat. No. 3,955,279 with a more strongly curved contour, or U.S. Pat. No. 4,489,493 with a smaller angle between the locating surfaces and the longitudinal axis would result in a more precise lateral position, but also in lower strength of the rearwards extending corners of the nose part, and a larger variation in longitudinal position with rise of inadequate support for the chain when traversing the joint.
It is also previously known to make the contour of the joint unsymmetrical with respect to the longitudinal axis of the guidebar, to make the joints in the two outer layers or two outer plates at the same guidebar edge longitudinally offset, giving the chain support on at least one side all the way as shown in U.S. Pat. No. 4,259,783 and Swedish Patent 433,468. These designs imply, however, that some of the rivets only pass through one of the outer plates of the nose part, leading to a high risk of a buckled or twisted joint with the nose part and the guidebar body in different planes, especially if a nose part is replaced with hand riveting in primitive conditions. This would be a safety problem, because the drive links of the saw chain could become stuck or expelled when traversing the joint.
The present invention concerns a guidebar with replaceable nose part, where the joint between the guidebar body and the nose part has been shaped with such a contour, that the contour may be blanked without a finishing operation; that the nose part and the guidebar body can be made from plates which have been integral but separated by blanking; that the chain is always supported on at least one side; that the corners of the nose part have adequate strength; and that the nose part and the guidebar body will always be in the same plane even after a primitively performed riveting.
In more detail, the invention is characterized by the rear contour of the outer plates of the nose part being shaped to comprise two rearwardly pointing extensions and between them one forwardly tapering recess with at least partly straight locating surfaces for contact with the guidebar body. The locating surfaces of said recess are preferably so oriented that they form an acute angle with the longitudinal axis of the guidebar between 10 degrees and 40 degrees.